The present invention relates to snap action thermal measurement devices and methods, and in particular to snap action thermal measurement devices formed as micro-machined electromechanical structures (MEMS).
Various temperature sensors are known in the art. Such sensors are used in various measurement and control applications. For example, thermocouples, resistive thermal devices (RTDs) and thermistors are used for measuring temperature in various applications. Such sensors provide an electrical analog signal, such as a voltage or a resistance, which changes as a function of temperature. Monolithic temperature sensors are also known. For example, a diode connected bipolar transistor can be used for temperature sensing. More specifically, a standard bipolar transistor can be configured with the base and emitter terminals shorted together. With such a configuration, the base collector junction forms a diode. When electrical power is applied, the voltage drop across the base collector junction varies relatively linearly as a function of temperature. Thus, such diode connected bipolar transistors have been known to be incorporated into various integrated circuits for temperature sensing.
Although the above described devices are useful in providing relatively accurate temperature measurements, they are generally not used in control applications to control electrical equipment. In such control applications various types of precision thermostats are used. The thermal switch is one form of precision thermostat used in control applications to switch on or off heaters, fans, and other electrical equipment at specific temperatures. Such temperature switches typically consist of a sensing element which provides a displacement as a function of temperature and a pair of electrical contacts. The sensing element is typically mechanically interlocked with the pair of electrical contacts to either make or break the electrical contacts at predetermined temperature set points. The temperature set points are defined by the particular sensing element utilized.
Various types of sensing elements are known which provide a displacement as a function of temperature. For example, mercury bulbs, magnets and bimetallic elements are known to be used in such temperature switches.
Mercury bulb thermal sensors have a mercury filled bulb and an attached glass capillary tube which acts as an expansion chamber. Two electrical conductors are disposed within the capillary at a predetermined distance apart. The electrical conductors act as an open contact. As temperature increases, the mercury expands in the capillary tube until the electrical conductors are shorted by the mercury forming a continuous electrical path. The temperature at which the mercury shorts the electrical conductors is a function of the separation distance of the conductors.
Magnetic reed switches have also been known to be used as temperature sensors in various thermal switches. Such reed switch sensors generally have a pair of toroidal magnets separated by a ferrite collar and a pair of reed contacts. At a critical temperature known as the Curie point, the ferrite collar changes from a state of low reluctance to high reluctance to allow the reed contacts to open.
Mercury bulb and magnetic reed thermal switches have known problems associated with them. More specifically, many of such switches are generally known to be intolerant of external forces, such as vibration and acceleration forces. Consequently, such thermal switches are generally not suitable for use in various applications, for example, in an aircraft.
Bi-metallic thermal switch elements typically consist of two strips of materials having different rates of thermal expansion fused into one bi-metallic disc-shaped element. Precise physical shaping of the disc element and unequal expansion of the two materials cause the element to change shape rapidly at a predetermined set-point temperature. The change in shape of the bi-metal disc is thus used to activate a mechanical switch. The bimetallic disc element is mechanically interlocked with a pair of electrical contacts such that the rapid change in shape can be used to displace one or both of the electrical contacts to either make or break an electrical circuit.
The critical bimetallic disc element is difficult to manufacture at high yield with predictable thermal switching characteristics. This unpredictability results in a need for costly, extensive testing to determine the set-point and hysteretic switching characteristics of each individual disc element. In addition, because the bi-metallic disc elements are fabricated by stressing a deformable or ductile metal beyond its elastic limit, which permanently deforms the material. The material, when the stress is removed, slowly relaxes toward its pre-stressed condition, which alters the temperature response characteristics. Thus, drift or xe2x80x9ccreepxe2x80x9d in the temperature switching characteristics can result over time. Next generation markets for thermal switches will require products with increased reliability and stability.
Furthermore, the bi-metallic disc element is by nature relatively large. Therefore, these thermal switches are relatively large and are not suitable for use in various applications where space is rather limited. Next generation thermal switches will require a reduction in size over the current state of the art.
Moreover, thermal switches actuated by the various sensing elements discussed above are normally assembled from discrete components. As such, the assembly cost of such temperature switches increases the overall manufacturing cost.
Another problem with such known thermal switches relates to calibration. More specifically, such known thermal switches generally cannot be calibrated by the end user. Thus, such known temperature switches must be removed and replaced if the calibration drifts, which greatly increases the cost to the end user.
Monolithic micromachined thermal switches have been developed in the past that obviate the necessity of assembling discrete components. These monolithic micromachined structures also allow the thermal switch to be disposed in a relatively small package. One example is a thermal switch described by co-owned U.S. Pat. No. 5,463,233 entitled, MICROMACHINED THERMAL SWITCH, issued to Brian Norling on Oct. 31, 1995, which is incorporated herein by reference, wherein a thermal switch includes a bi-metallic cantilever beam element operatively coupled to a pair of electrical contacts. A biasing force such as an electrostatic force is applied to the switch to provide snap action of the electrical contacts in both the opening and closing directions which enables the temperature set point to be adjusted by varying electrostatic force biasing voltage.
Although many of these known thermal switches are useful and effective in current applications, next generation applications will require products of reduced size with increased reliability and stability beyond the capabilities of the current state of the art.
The present invention provides a small and inexpensive snap action thermal measurement device which can retain its original set point over long operating life and large temperature excursions by providing a thermal switch actuator fabricated from non-ductile materials, in contrast to the prior art devices and methods.
The apparatus and method of the present invention provide a simplified snap-action micromachined thermal switch that eliminates any requirement for electrical bias to prevent arcing. The apparatus of the invention is a thermal switch actuator fabricated from non-ductile materials such as silicon, glass, silicon oxide, tungsten, and other suitable materials using MEMS techniques that replaces the bimetallic disc thermal actuator described above. The use of non-ductile materials solves the lifetime creep problems, while the use of MEMS manufactured sensors addresses size and cost issues. The resulting thermal switch is alternatively configured to drive a solid state relay or a transistor.
According to one aspect of the invention, the bimodal thermal actuator includes an actuator base structure formed of a first substantially non-ductile material having a first coefficient of thermal expansion, the actuator base structure being formed with a relatively mobile portion and a substantially stable mounting portion extending therefrom; a cooperating thermal driver structure formed of a second substantially non-ductile material and having a second coefficient of thermal expansion different from the first coefficient of thermal expansion, the thermal driver structure being joined to at least a portion of the mobile portion of the actuator base structure; and an electrical conductor portion formed on the mobile portion of the actuator base structure.
According to another aspect of the invention, at least one of the first and second substantially non-ductile materials of the bimodal thermal actuator is selected from a family of materials having a high ultimate strength and a high shear modulus of elasticity.
According to another aspect of the invention, the mobile portion of the actuator base structure of the bimodal thermal actuator is formed in an arcuate shape.
According to another aspect of the invention, the cooperating thermal driver structure of the bimodal thermal actuator is formed as a thin layer of the second substantially non-ductile material joined to the mobile portion of the actuator base structure adjacent to the substantially stable mounting portion thereof.
According to another aspect of the invention, the electrical conductor portion of the bimodal thermal actuator is formed as a portion of the mobile portion that is doped with electrically conductive material.
According to another aspect of the invention, the electrical conductor portion of the bimodal thermal actuator is formed as a metallic electrode at a central portion of the mobile portion.
According to another aspect of the invention, the invention provides a micromachined thermal switch that further includes a support base having an upright mesa and an electrode formed on one surface; and the mounting portion of the bimodal thermal actuator is coupled to the mesa with the electrical conductor portion of the mobile portion aligned with the electrode on the support base. According to other aspects of the invention, the support base includes two upright mesas with the electrode formed on the surface in between. The bimodal thermal actuator is suspended from the two mesas with the electrical conductor portion provided at the center of the mobile portion in alignment with the electrode on the support base.
According to still other aspects of the invention, the invention provides a method for determining temperature, the method providing joining together two substantially non-ductile materials having different coefficients of thermal expansion along a common surface in a bimodal thermal actuator having an actuator portion being mobile relative to a mounting portion and having an electrically conductive area situated at one surface thereof; and wherein the relatively mobile actuator portion is further disposed subsequently in a plurality of stable relationships to the mounting portion as a function of sensed temperature, a first stable relationship of the relatively mobile actuator portion to the mounting portion positioning the electrically conductive area in contact with an electrode, and a second stable relationship of the relatively mobile actuator portion to the mounting portion spacing the electrically conductive area away from the electrode.
According to another aspect of the method of the invention, the first stable relationship places the electrically conductive area of the relatively mobile actuator portion on a first side of the mounting portion, and the second stable relationship places the electrically conductive area of the relatively mobile actuator portion on a second side of the mounting portion opposite from the first side.
According to another aspect of the method of the invention, the method further provides joining the mounting portion of the bimodal thermal actuator in relationship to a support structure including the electrode.
According to yet another aspect of the method of the invention, the method further provides forming the relatively mobile actuator portion in an arcuate configuration extending from the mounting portion.
According to still another aspect of the method of the invention, the method further provides forming the mounting portion as a pair of spaced apart mounting portions; and forming the relatively mobile actuator portion in an arcuate configuration extending between the pair of spaced apart mounting portions.